Culvert design over the last 20 to 30 years has advanced considerably, particularly with respect to large diameter culverts, box culverts and re-entrant arch shaped culverts. Corrugated metal culverts of large diameter have gained general acceptance for use under roadways, railways and the like. Circular culverts have significant drawbacks associated with waterway installations because the stream bed must be disturbed. In order to reduce the impact on the stream bed, arch structures are preferred. The arch structure has an open base and as such relies on a set of design requirements different from circular culverts for supporting the overbearing load. Arch structures have a large radius crown and usually have straight sides as associated with the box culvert. Box culverts are particularly useful in meeting a need for structures with large cross-sectional areas for water conveyance with limited vertical clearance. Normally, metal box culverts are made of either aluminum or steel. Usually the plate which is used in the culverts is corrugated to strengthen the design. The corrugated plate, particularly if it is aluminum, is usually strengthened by reinforcing ribs or the like at intervals along the culvert length.
An example of this type of rib reinforced aluminum culvert design is disclosed in U.S. Pat. No. 4,141,666 issued to Kaiser Aluminum and Chemical Corporation. The use of reinforcing members on the outside of the box culvert provides for the necessary load carrying capacity. However, sections between the reinforcing members are considerably weaker and hence, when loaded, there is a differential deflection or undulating effect along the length of the culvert. To reduce this problem, unitary extrusions are secured to the inside of the culvert to reduce undulation, particularly along the crown and base portions. It is understood however that when box culverts are used over stream beds or the like, it is not desirable to include inside the culvert any attachments particularly used in reinforcing culvert design structures because they tend to be destroyed during ice flows and floods.
The use of strengthening ribs has also been applied to metal box culverts, such as disclosed in U.S. Pat. No. 4,318,635. Multiple arched-shaped reinforcing ribs are applied to the culvert interior and/or exterior to provide for reinforcement in the sides, crown and intermediate haunch portions. However, such spaced apart reinforcing ribs, although they enhance the strength of the structure to resist load do not overcome undulation problems in the structure and can add unnecessary weight to the structure by virtue of superfluous reinforcement.
U.S. Pat. No. 5,118,218 discloses a box culvert design which does not involve the use of reinforcing members. Instead, the sheets of metal used in constructing the culvert have exceptionally deep corrugations of a depth in the range of 100 to 150 nm with a pitch in the range from 300 to 450 mm. By using significantly thicker material in the crown portions of the culvert and perhaps as well in the haunch portions of the culvert, significant loads can be carried by the culvert design. However, significant limitations exist with respect to the crown in regard to radius of curvature. Radius of curvature of less than 1 m is avoided with steel because of the significant potential for microcracking and fissuring due to cold working or strain hardening when bending the steel to the desired radius of curvature. With aluminum, the shorter radius of curvature is avoided because of the possible permanent deformation of the cross-section during forming due to the lower modulus of elasticity. Furthermore, the use of thicker metal in the crown or haunch portions of the culvert without reinforcing can add considerably to the overall weight of the structure in order to carry design loads. Metal box culverts are usually designed using plastic theory rather than elastic theory. It is generally accepted that one of the significant drawbacks with existing box culvert designs is that one cannot take full advantage of the plastic theory.
The elastic theory of design requires that the design be based on the allowable stress method whereas the plastic theory of design considers the greatest load which can be carried by the structure when acting as a unitary structure. The advantage in a plastic design procedure is that there is a possibility of significant saving in the amount of metal required and hence, permit culvert design which can give a more accurate estimate of the amount of load that a structure can support. Metal box culverts are often subject to large stresses which are difficult to predict such as those caused by and erection of the structure and subsequent structure settlement. Plastic design criteria however provides for such situations by permitting plastic deformation in the structure. The plastic moment is generally understood to be the moment which will produce plasticity in a member of the box culvert and create a plastic hinge. In design of metal box culverts, plastic moments are distributed between the crown and the haunch. Theoretically, this distribution could be as unbalanced as 0 to the haunch and 100% to the crown which would resemble a simple supported beam. However, current practice in design restricts the distribution to 45% minimum and 70% maximum to the crown. Current design specifications such as AASHTO publish the required plastic moment capacity for the crown and haunch of metal box culverts. These specifications cover spans from 2.5 m to 8 m and cover depths of overload from 0.4 m to 1.5 m In the metal box culvert designs which are reinforced with metal stiffening ribs, there is a complex interaction of the stiffener ribs with the corrugated plate. The section properties at each metal rib provide greater inertia or stiffness at the ribs. The corrugated plate functions as a membrane between the ribs and transfers loads to the ribs. The corrugated plate between the ribs is then subjected to axial stress on two axis or about two axis that is circumferential and transverse. Because of this complex interaction, a rational analysis is difficult and hence there is a need to move towards the plastic design of a section with uniform stiffness and subject to stress on only one axis.
In unreinforced metal box culverts, the difference in plastic moment between the crown and haunch is achieved by changing the thickness of the corrugated plate. In the case of the shallow depth of cover, the plastic moment at the crown is usually much greater than the plastic moment at the haunch resulting in a crown plate thickness usually greater than the haunch plate thickness. In the case of deep covers over the culvert, say in the range of 1.5 m, the plastic moment at the crown can be equal to the plastic moment of the haunch. In ureinforced metal box culvert design the selection of corrugation profile and metal thickness is based on providing the appropriate plastic moment resistance at the haunch or crown. At all other locations more material is provided than necessary and hence, the significant addition of weight to the structure as well as increased costs in manufacture and material costs.
It has also been found that the 8 m limitation with respect to span of existing metal box culvert designs is overly restrictive for the culvert designer. There are several situations where a span of 8 m or greater would be desired. However, with existing culvert design, such spans cannot be achieved. Any attempt to reduce the load above the culvert, such as the use of concrete slabs at surface level or below surface level, but spaced above the culvert crown, considerably increases the total cost of the metal box culvert installation, particularly in regions where concrete may not be readily available. Concrete has also been used to reinforce culvert bases such as disclosed in German patent application 26 57 229. The concrete is retained in position by an outer skin of corrugated metal spaced from the culvert by the concrete thickness. However, the concrete reduces the ductility of the structure and prevents thereby the redistribution of plastic moments and the application of plastic theory.
The continuously reinforced box culvert design of this invention has significant advantages over the prior art and allows one to achieve plastic design procedures while avoiding the problems associated with the unreinforced or reinforced culvert designs.